A wide variety of foods contain an introduced gas such as air, nitrogen, carbon dioxide or nitrous oxide. Typically, food foams are unstable over periods of time in excess of several days (i.e. bubbles will grow and the foam will collapse) unless the product is frozen (e.g. ice cream) or unless the continuous phase of the product is gelled (e.g. mousse).
Currently, it is difficult to prepare an aerated product where the foam is stable over a shelf life of several weeks which, with some agitation if necessary, will then flow. There are two aspects that need to be considered: (a) the stability of the bubbles over several weeks; (b) prevention of the rise of air bubbles to the surface due to the buoyancy difference between the two phases (termed creaming). Creaming can lead to a poor appearance of a product and loss of performance. For example, if a foam creams then the bubbles can become less stable and air can be lost. Furthermore, a “phase separated” product can look much less desirable to the consumer.
Ideally, such an aerated product would contain a foam where the bubbles remain stable to growth and creaming, yet on application of a relatively small amount of agitation (e.g. shaking, stirring or sucking), the product will flow.
There are a number of mechanisms that degrade the quality of an aerated product: Disproportionation and coalescence lead to bubble growth, changing product properties, such as its texture and physical appearance. Creaming (due to the buoyancy of the air bubbles) leads to vertical phase separation in the container resulting in a large proportion of bubbles close to the upper surface and the depletion of bubbles at the bottom. There are aerated food products where creaming is desirable, e.g. the foam on the surface of beer. However, for aerated products requiring a foam life-time beyond a few minutes or hours (e.g. those requiring a shelf-life), creaming leads to an undesirable appearance. It can also lead to subsequent air loss due to the closer packing of the bubbles in the foam and the foam collapse that may result there from.
To design a product were creaming is prevented and the bubbles within the foam remain stable over an extended shelf-life (from 1 week to at least 3 or more weeks at chill), there are two pre-requisites: The continuous phase requires either an apparent yield stress or must be visco-elastic to prevent bubbles rising due to buoyancy. The bubbles within the foam must be stable to disproportionation and coalescence throughout the product life-time.